Thrombin is the central enzyme involved in blood coagulation. This enzyme ia also a potent and physiologic activator of platelets. The regulation of prothrombin activation on the platelet surface and the mechanism of thrombin action on platelets are important subjects in hemostasis and are under active investigation. Two thrombin-reactive polypeptides of 74 kDa and 55kDa, which are located on the surface of human platelets, have been isolated in this laboratory. These polypeptides specifically interact with thrombin and modulate its reactivity on platelets, fibrinogen and N-benzoylarginyl ethyl ester. Two immunochemically identical proteins with similar properties on thrombin reactivity have also been detected and isolated from fresh human plasma. Recent studies show that these proteins are immunochemically related to coagulation factor V. The role of FV in the regulation of thrombin generation and protein C activation by thrombin are known. Although important differences between the 74kDa and 55 kDa proteins and FV exist, there are some similarities between these polypeptides and FV activation products by thrombin. The immunologic relatedness further suggest some structural similarities between FV and these polypeptides. Studies are proposed to explore the role of 74 kDa and 55 kDa polypeptides in (a) mediating the activation of platelets by thrombin utilizing the Fab fragments of antibodies to these proteins, (b) modulating the reactivity of thrombin by directly interacting with the enzyme and (c) regulating thrombin generation in the coagulation cascade by influencing FV interactions with platelets and protein C activation by thrombin. Factor V, FX and protein C will be isolated from fresh plasma by established procedures. The effects of the 74 kDa and 55 kDa polypeptides on FVa binding to platelets, FXa binding to platelet-bound FVa and the cofactor activity of FVa in protein C activation of thrombin will be evaluated. In addition the synthesis, postranslational processing and secretion of these proteins will be investigated in liver cells. These data will also indicate whether they have an origin common to FV. The results of these studies are essential for the understanding of the basic principles involved in hemostasis and thrombosis.